It sounds like the wifi of the modem is turned off or broken. Try plugging a device directly in the modem (i.e. by a cat5 cable) and see if you can get a stream.
If you have the little NBN modem box or are FTTP try plugging a pc directly into that and bypassing the optus modem all together. You may get NBNCo direct that way.
But otherwise its unlikely anyone can help until you get the replacement modem.